Use of electrical power generators is quite extensive with a wide range of applications. It will be understood that these electrical power generators typically take drive from a primary mover in the form of an engine and therefore create electrical power for control and actuation of that engine as well as for other functions ancillary to the engine or vehicle or structure in which the engine is located. With regard to gas turbine engines it will be appreciated that a significant number of such engines are utilised in aerospace applications. In such circumstances a gas turbine engine is utilised in order to drive the electrical power generator to provide electrical power to both the engine for control and actuator functions as well as with respect to providing electrical power for the aircraft itself.
Electrical power generators are an inherent feature of modern aero engines to generate electrical power for local engine control loads and also to provide electrical power for wider demands within the aircraft in general. It is known that parallel operation of generators can bring benefits in terms of system efficiency, weight and availability. Having a number of electrical power generators ensures greater reliability of electrical power supply should one generator or its system fail and also in terms of meeting the variable electrical demand levels in an aircraft at different stages of aircraft cycles, e.g. take-off, landing and cruising. In order to achieve parallel operation of generators a suitable control method needs to be adapted to prevent conflicts in electrical power provision occurring between the different electrical power generators. Such previous electrical generator control arrangements have required that one of the generators operates with a voltage control and the other electrical generator or generators with electrical current control. FIG. 1 below provides a schematic illustration of the differing control mechanisms in order to achieve a steady state point for an electrical power generator arrangement or system. This steady point is depicted as 1 in the graphic representation. This target value 1 as indicated is achieved by respective electrical generators operating to achieve validation controlled via voltage referencing and via electrical current referencing respectively. In such circumstances, as can be seen, a reference or command voltage Vcmd is compared in a voltage comparator 2 with an output voltage Vout. The comparator 2 will generally subtract the output voltage Vcmd so that any deviation or error is determined by a voltage transfer function and machine characteristic in order to adjust the electrical generator to maintain the desired output voltage at the reference value Vcmd. Similarly with regard to electrical current control, a current comparator 3 compares a reference or desired electrical current Icmd with an output electrical current Iout. This comparison is normally by subtraction of one from the other such that any error is determined by a current control transfer function and machine characteristic such that the electrical generator thereby controlled is altered in terms of its performance in order to ensure that Iout is equal to Icmd.
It will be understood that the voltage control and current control use different control loops and therefore require separate implementation. However, parallel operation of the electrical generators may be temporarily abandoned, either specifically to meet electrical demands or under fault conditions when one or more of the electrical generators may malfunction. In such circumstances the electrical generators must be able to operate independently. Furthermore, it is important that the electrical system voltage is maintained at all times so that it is necessary for all electrical generators to be able to revert to voltage control mode. In such circumstances at least some of the electrical generators must be able to operate with both voltage and electrical current control modes except the one normally voltage controlled. It will also be understood that the two modes of control operation must be able to be switched without interrupting the normal operation of the generators. In such circumstances, when parallel operation is abandoned an electrical generator operating under electrical current control should be switched to voltage control quickly, otherwise a large unpredictable voltage transient may result.